Upgrade postgresql chart requirement to version >= 8.1.0

Summary

As described in #1987 (closed), we'd like to be able to set the priorityClassName for the postgresql sub-chart to ensure the postgresql pods can be given higher priority over other non-AZ-bound pods. The postgresql sub-chart added support for configuring priorityClassName in version 8.1.0; the current version referenced by the gitlab helm chart] is version 7.7.3.

Prerequisites

As discussed in #1987, this upgrade should wait until !1253 (closed) is merged and #1970 (closed) is closed (migrating from the deprecated stable/postgresql chart to the bitnami chart).